Coolant tubes for heat exchangers have generally flat side walls connected by rounded bight portions along their opposite longitudinal edges. In a heat exchanger the tubes are arranged interdigitally with corrugated fin strips with the side walls soldered to the successive crests of the fin strips. Such tubes are usually rolled from flat strip stock into tubes of circular cross section which are butt welded along their abutting edges and then flattened into the desired finished shape. This method of forming such tubes has several inherent disadvantages. When the strip stock is rolled into circular shape and then flattened, the metal is subjected to considerable cold working which can result in defects such as cracks in the finished tube. In addition, the strip stock from which such tubes are rolled frequently has a thickness of less than 0.010"; consequently, prior to welding the rolled tube is quite flimsy and flexible and it is very difficult to align the edges exactly in opposed abutting relation for welding.
The primary object of this invention is to provide a method and apparatus for rolling flat metal strip stock into generally flat tubes at a high rate of speed and in a manner which reduces cold working of the metal to a minimum.
A further object of the invention is to provide a tube mill wherein the strip stock is bent along an extremely accurate center line which is thereafter utilized to center the strip as it is directed through the successive forming rolls.
Another object of the invention is to provide a guide mechanism which engages each of the opposed free edges of the strip on laterally opposite sides thereof to maintain them in perfect alignment as the strip is directed through the welding station.
U.S. Pat. No. 4,595,135 is directed to an improved method wherein flat strip stock is rolled into a flat tube while maintaining the side walls of the tube in a generally flat condition throughout the rolling operation. As a result, the metal is subjected to a minimum of cold working. Furthermore, since the side walls of the tube are maintained substantially flat, the cross section of the tube prior to welding presents a relatively rigid structure as compared to a circular cross section. Thus the operation of exactly aligning the opposed free edges for welding is rendered relatively simple More specifically, the method comprises the steps of first rolling the opposite edges of the strip into a curved configuration having a radius corresponding to the radius of curvature desired on the rounded longitudinally welded edge of the tube, then progressively rolling the central portion of the strip into the rounded configuration desired along the other longitudinal edge of the tube while utilizing the rounded free edge portions and the center bend to accurately center the strip as it advances through the successive forming rolls and, thereafter heating the spaced free edges of the rolled strip to a temperature at which they are adapted to be fused together and directing them through a guide located upstream from and directly adjacent a pair of squeeze rolls for effecting a weld seam therebetween, the guide being designed to engage each free edge portion of the strip on laterally opposite sides thereof.
The method and apparatus disclosed in U.S. Pat. No. 4,595,135 has been found to be capable of rolling metal strip stock into generally flat tubes at a high rate of speed and in a manner which reduces cold working of the metal to a minimum; wherein the strip stock is bent along an extremely accurate center line which is thereafter utilized to center the strip as it is directed through the successive forming rolls; and which utilizes a guide mechanism which engages each of the opposed free edges of the strip on laterally opposites sides thereof to maintain them in perfect alignment as the strip is directed through the welding station.
However, it has been found that the guide mechanism must be accurately positioned to guide the flat strip stock into the first rolling step and, unless this is done carefully, the extremely accurate center line will not be maintained. As a result, the final step of welding is made more difficult and a consistently sound weld seam may not be assured.
Accordingly, among the objectives of the present invention are to provide a method and apparatus for rolling flat metal strip stock into generally flat tubes wherein the strip stock is bent along ,an extremely accurate center line without the necessity of carefully positioning and guiding the strip stock into the first step wherein the opposite edges of the strip are rolled into the final radius corresponding to the radius of curvature desired on the rounded longitudinally welded edge of the tube.
In accordance with the invention, an additional rolling step is provided wherein initially the opposite edges of the flat strip are rolled by restraining the free ends of the side edges and bending the side edges into a configuration comprising flat side edge portions connected by a radius portion to the central flat portion of the strip and extending at an acute angle to the flat central portion of the strip This partially curved configuration serves as a guide to accurately position the strip as it moves into the next step of rolling the opposite flat edge portions of the strip into a curved configuration having a radius corresponding to the radius of curvature desired on the rounded longitudinally welded edge of the tube. The strip is then moved through the successive steps of progressively rolling the central portion of the strip into the rounded configuration desired along the other longitudinal edge of the tube while utilizing the rounded free edge portions and the center bend to accurately center the strip as it advances through the successive forming rolls and, thereafter heating the spaced free edges of the rolled strip to a temperature at which they are adapted to be fused together and directing them through a guide located upstream from and directly adjacent a pair of squeeze rolls for effecting a weld seam therebetween, the guide being designed to engage each free edge portion of the strip on laterally opposite sides thereof.